Methylation of histone H3 at lysine 27 has an important role in transcription repression and it is catalyzed by G9a and Polycomb Groups enzymes such as EZH2. H3 (mono-methyl K27), a modification enriched at periocentromic heterochromatin, is broadly distributed throughout all euchromatic sites and participates in silencing gene expression. H3 (di-methyl K27) and H3 (tri-methyl K27) are the facultative heterochromatin mark which promotes the recruitment of PcG proteins for gene silencing.